Ps3 Nat 3 Help Please ?

Recently i noticed on MW2 when i tried to join my friends games it rarely let me i've been told this is because on my network settings it says Nat 3 when it should be Nat 2 but I don't know how to change it
Please Help
Thanks

Suggestion:

You have to go into router settings and set your wireless or wired router to the PS3 setting required. Are you wired or wireless? What kind of router are you using? Who is your internet provider? More details please.